29th International Cosmic Ray Conference Pune (2005) 6, 21–24

Angular Resolution of the GRAPES-3 EAS Array for UHE
Gamma-Ray Astronomy
P.K. Mohanty , S.K. Gupta , Y. Aikawa , N.V. Gopalakrishnan , Y. Hayashi, N. Ito , A.

Iyer , A. Jain , P. Jagadeesan , A.V. John , S. Karthikeyan  , S. Kawakami , T. Matsuyama ,D.K. Mohanty , S.D. Morris , T. Nonaka , A.
 Oshima , B.S. Rao , K.C. Ravindran ,K. Sivaprasad , B.V. Sreekantan , H. Tanaka , S.C. Tonwar , K. Viswanathan and T.
Yoshikoshi(a) Tata Institute of Fundamental Research, Homi Bhabha Road,Mumbai 400005, India(b) Graduate School of Science, Osaka City University, Osaka 558-8585,Japan

Presenter: P.K. Mohanty (pkm@tifr.res.in), ind-mohanty-PK-abs1-he11-poster
Observations on cosmic sources of ultra high energy (UHE) gamma rays provide information on the origin ofcosmic rays and their acceleration mechanisms. In order to detect a UHE gamma ray source with its extremely
low flux against a large isotropic cosmic ray background, an Extensive Air Shower (EAS) array with very large
collection area and high angular resolution is necessary. The GRAPES-3 EAS array operating at Ooty covering
an area of  18000 m with  300 fast scintillation detectors is being used for studies on UHE gamma-ray
sources. Here we have described the method of reconstruction of the arrival angle of showers and determinationof the angular resolution of the GRAPES-3 array.

1.

Introduction

GRAPES-3 is a high density air shower array designed to measure both densities and relative arrival times ofshower particles to determine the energy and incident direction of primary cosmic ray particle which initiatesa shower entering into the atmosphere. The array records ADCs for density and TDCs for relative arrival timemeasurement of shower particles. The details of the experiment with its shower detectors, shower trigger anddata acquisition is described elsewhere [1]. The array has the following features required to study UHE souces.
1. High trigger rate  25 Hz with total recorded showers per day  2.2 million in the primary energy range 10TeV - 10 PeV
2. Trigger efficiency for primary   90  for E  30 TeV.
3. Arrays association with a large area (560 ) muon detector which detects the muon component of theshower [2]. This helps to reject large fraction of charged cosmic rays on the basis of muon content of theshower and enhances the signal to background ratio.
In this paper, we have studied the angular resolution of the array.
The angular resolution in zenith angle  is given by    
  !#"$Where is the uncertainty in measuring the arrival times
D is average distance between two detectors
N is the number of detectors used to estimate the angle

(1)

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2.

P.K. Mohanty et al.

Time Resolution of the GRAPES-3 Showers Detectors

Figure 1. Distribution of arrival time difference of showers Figure 2. Arrival time difference when each detector has
more than 3 particles
between two closely placed shower detectors

The time resolution which is contributed by finite thickness of shower disk and instrumental timing error
has been measured placing two shower detectors side by side. The width of the time difference distribution
for all particle density shown in figure 1 is 5.5ns and for particle density % 3 shown in figure 2 is 2.9 ns. The
time resolution of individual detector for all particle density is 5.5ns/ & $ = 3.9 ns and for particle % 3 is 2.9/ & $= 2 ns. At higher particle density both the detectors
sample the showers mainly near the core where showerthickness is small. The main contribution to comes from instrumental time resolution.

Using
measured by above method, a simple Monte-Carlo simulation has been performed to get a roughidea about the angular resolution which can be achieved by
 GRAPES-3 array. Taking  = 25 ' , D = 8 meter (separation between two GRAPES-3 detectors, N=20 and = 3.9 ns, the computed angular resolution is 1.8 ' .

3.

Reconstruction of Arrival Direction of Shower

The arrival direction of shower characterized by zenith angle  and azimuth angle ( has been reconstructed byfitting a plane to the observed relative arrival times of shower particles at individual detectors using least square
method. Before doing the fit, these arrival times have been corrected for the time offsets for the individualdetectors which arise due to difference in photomultiplier transit time, electronic propagation delay etc. Thesetime offsets have been determined from the shower data itself by a method developed and used by KGF group[3].
The angle is reconstructed for those showers, for which highest recorded density detector is on and within thesecond last ring of the array. This condition is imposed because the shower which gives highest density in thelast ring, its core might be landed out side the array boundary and with partial information available, the angle
determination can be highly inaccurate. This selection rejects 21  of showers from angle reconstruction.

The  and ( distributions are shown in figure 3 and 4 with the analysis of data from 1-10th April 2005. (is measured with reference to north direction and clockwise. Figure 4a - 4d show ( distribution for different
zenith angle ranges. The ( distribution is flat in figure 4a which is expected due to the isotropic arrival of
cosmic rays, whereas deviations seen in figure 4b - 4d from flat distribution. The deviation is quite large inAngular Resolution of the GRAPES-3 EAS Array

23Figure 3. Zenith angle distribution of showers
Figure 4. Azimuth angle ) distribution for different zenith
angle ranges. A large decrease is seen at higher zenith angle
from expected flat distribution

figure 4d for % 30 ' . The increase in the deviation for higher zenith angle shows the effect of ground slope on
the shower trigger. The array has slope mainly in the east-west direction. A more detail analysis is in progressto understand this ( asymmetry quantitatively though Monte-Carlo simulation.

4.

Angular Resolution by Even-Odd Array Method

Here two independent estimate of the angle of the shower has been compared to find the angular resolution,
one calculated using even numbered detectors of the array and the other using the odd numbered detectors.
The angular resolution is calculated dividing by & $ . & $ with the space angle distribution between the twoestimates. one & $ comes because we are comparing two distribution where the error adds quadratically andthe other & $ comes because we are using nearly half of the detectors to calculate the angle.
From figure 5 it can been seen than the angular resolution is better with more number detectors used in angle
fit. The median value of angular resolution is 2 ' for NFIT % 10 and it improves to 1 ' for NFIT % 40.

5.

Discussion and Conclusions

The above analysis gives a rough estimate of angular resolution determined by plane fit. The angular resolution
improves with NFIT. A More detail analysis is in progress which may help to improve further the angularresolution taking into account the shower curvature correction and electronic slewing effect. We are trying tosee the Moon shadow which will verify our calculation of angular resolution.

24P.K. Mohanty et al.

Figure 5. space angle distribution between Even and Odd array as a function of number detectors participated in the angle
fit (NFIT). The width of the distributions become narrower from (a) to (d) which tells that the angular resolution is betterwith more number of detectors.

6.

Acknowledgments

We thank D.B. Arjunan, A.A. Basha, G.P. Francis,I.M. Haroon, V. Jeyakumar,K. Manjunath, B. Rajesh, K.
Ramadass, C. Ravindran, and V. Viswanathan for help in construction and operation of GRAPES-3 experiment.
We acknowledge partial financial support from Ministry of Education, Government of Japan.

References[1] S.K. Gupta et al., Nucl. Instr. & Meth. A 540, 311 (2005).
[2] Y. Hayashi et al., Nucl. Instr. & Meth. A 545, 643 (2005).
[3] B.S. Acharya et al., J.Phys. G: Nucl. Part. Phys. 19(1993) 1053-1068